Pipelines are long, hollow, flexible tubes of metal. When one end of a long pipeline lying horizontally on a flat surface is raised vertically, the pipe bends due to the weight of the unsupported segment. The maximum bending stress occurs in the bend near or at the horizontal surface. This bend is commonly referred to as the sagbend. As the end of the pipe is raised higher, the bending stress increases in the sagbend. Depending partially upon the pipe diameter, the wall thickness, the tensile strength of the metal and the external pressure on the pipe, the pipe will collapse when the compressive and tensile forces developed along the bend exceed the physical strength of the metal walls. The collapse may result in tearing and otherwise rupturing the pipe wall. When this occurs under water, as in a submarine pipe laying operation, it is known as a wet buckle. The free end of an anchored horizontal pipe can be raised higher before collapse occurs, if the pipe is stretched; e.g., kept under tension. Loss of the ability to keep the pipe under tension, due possibly to the failure of a barge anchor to hold, may result in a wet buckle of the pipe. The high velocity (up to 100 fps) water which enters through the rupture may fill a significant portion of the pipe before corrective action can be taken. Further, the water introduces undesirable and difficult to remove silt, sand and stones into the line. Before laying can be resumed, the pipe is commonly blown clear of water so that the ruptured end can be picked up. Wet buckles thus can result in substantial delays.
Lines can also be ruptured or punctured by foreign objects away from the sagbend. Anchors of ships are an example of such foreign objects. Water rushing in through the puncture will reach the sagbend and begin to fill the vertical portion of the pipe. The additional unsupported weight will in turn lead to a wet buckle at the sagbend.
One method of burying pipe lying at the bottom of water is to remove the soil beneath the pipe with high-velocity water jets which suspend the soil particles in a water-soil slurry so that the pipe sinks into the suspended soil. An air-filled pipe, because of its light weight in the water, will not readily sink into the suspension. While water filling the pipe prior to trenching can reduce the time, effort, and expense of burying the pipe, the water may also lead to wet buckles as above noted.
A weight coating such as concrete may be applied to large-diameter pipe to cause it to sink in water. The weight handling ability of the pipe laying equipment and economic reasons normally limit the quantity of coating close to the minimum necessary to cause the pipe to be negatively buoyant in water. The air-filled pipe lying on the bottom of the water body then is near the density of the water and thus easily moved about by flow currents which can shed vorticies behind the pipe. These vorticies, if at one of the natural frequencies of the pipe, may induce vibratory motion in the pipe with resulting stresses spalling the concrete off the pipe, creating a buoyant pipe. Thus, it is desirable to water-fill pipe, yet somehow avoid the above-noted problems with wet buckles.
Of pertinance to the present invention are U.S. Pat. Nos. 3,425,453; 3,842,613; 3,788,084; 3,472,035; 3,656,310; 3,777,499; 3,525,266; 3,751,932; 3,890,693.
The present invention not only provides a solution to the above problem, but also includes other improvements as more particularly described hereinafter.